In the GSM system a 14 digit International Mobile station Equipment Identity (IMEI) code is defined to be stored in the mobile station. The value of the IMEI is intended to be unique for each mobile station, and is fixed during the manufacturing process. An associated check digit is calculated by a Luhn Algorithm (ISO/IEC 7812).
The IMEI is typically employed by the network operator to insure the validity of the mobile station, and the network operator will refuse service to a mobile station that does not have a valid IMEI. A purpose of the IMEI is to obtain information on the use of Public Land Mobile Network (PLMN) by subscribers for individual charging purposes.
As currently specified, the IMEI includes a six digit binary coded decimal (BCD) serial number (SNR) field, as well as a six digital type approval code (TAC) and a two digit final assembly code (FAC). However, with the explosive growth in mobile station usage, the six digits of the SNR field becomes insufficient for uniquely identifying mobile stations for large production volumes.